Many light detection systems require artificial illumination sources in order to ensure that the system has sufficient light for reliable performance. For example, a scanner may include an illumination source--to provide the necessary print-to-background contrast for forming image signals representative of a scanned original. A high contrast level enhances the signal-to-noise ratio (SNR) of image signals generated by one or more photodetectors. The enhanced SNR increases the accuracy of image reproduction. Moreover, high contrast levels also permit the scanning operation to be performed at a higher speed.
The selection of an appropriate illumination source for a light detection system depends upon a number of factors. A multi-feature photocopier typically utilizes a fluorescent lamp to permit high speed document reproduction. In comparison, illumination sources for a hand-held scanner must be compact and energy efficient.
An attractive approach to providing a compact, low energy illumination source for a detection system is an electroluminescent device that emits light from a planar surface in response to an excitation signal. Thick film and thin film electroluminescent devices are well known. One or more active layers may be sandwiched between two electrode layers connected to a voltage source. At least one of the electrode layers is transparent. When a potential difference is applied by means of the two electrode layers, energy is emitted at specific wavelengths that depend upon the selection of the active layers. The radiation passes through the transparent electrode and may be used to provide a generally uniform illumination field on a surface of interest, such as a document. A charge coupled device (CCD) array or other detector arrangement may be used to form signals representative of light scattered from the surface of interest. A concern is that the intensity of the illumination field drops off considerably with distance from the electroluminescent device. The reduced intensity may adversely affect performance of the detection system, particularly if the detector array is adjacent to the electroluminescent device in a surface-contact application, i.e., an application in which the device is brought into contact or near contact with the surface of interest. The radiating region of the electroluminescent device does not typically extend to the edge of the device, since the layers are hermetically sealed. Humidity and oxygen significantly reduce the useful life of most electroluminescent devices. Consequently, the edges of the device are sealed to exclude water and oxygen from the active layers. The seal reduces the susceptibility of the device to rapid degradation, but increases the distance between the radiating region and the field of view of the CCD array or other light detection device, as shown in FIG. 1. The seals 11 and 13 in FIG. 1 protect the electroluminescent layer stacks 15 and 17. Each layer stack is sandwiched between a pair of transparent substrates 19, 21, 23 and 25. A space between the two pairs of substrates allows light reflected from a document 27 to pass to an optical arrangement 29, as represented by the vertical line from the document to the optical arrangement. The presence of the seals 11 and 13 at the edges of the layer stacks 15 and 17 determines the minimum distance between the radiation regions of the layer stacks and the field of view of the optical arrangement 29.
U.S. Pat. No. 5,491,384 to Cheng et al. describes a light source for use in a surface-contact application, such as scanning a document for facsimile transmission. An electroluminescent source is formed between two water-proof layers. A narrow slot is then formed through the device. The device is fastened to a scanning face glass that is to be brought into contact with the surface of interest. Light reflected from the surface of interest passes through the narrow slot to a lens of a contact image sensor. Because the narrow slot is formed directly through the active layers, there is no sealant at the narrow slot. While this arrangement improves the intensity distribution into the line of sight of the sensor, it exposes the active layers to moisture and oxygen. What is needed is a device which provides substantially uniform illumination intensity over a region of interest to a light detection system in a compact and energy efficient manner. What is further needed is a method of fabricating such a device so as to protect the device from atmosphere-induced degradation.